The present invention relates to an apparatus for the granulometric analysis of aerosols by cascade impaction and uniform deposition having a conical air inlet fixed to a tight, hollow cylindrical chamber of revolution constituted by superimposing several stages in the form of hollow cylinders. Each stage is traversed by the air flow from the conical inlet and each stage has a circular plate perforated by holes located on concentric circles coaxial to said plate, the holes in each stage having identical diameters while the hole diameter progressively decreases in successive stages, as well as a collection disk positioned downstream of the perforated plate whose diameter is less than the internal diameter of the stage, incorporates means for ensuring the downwardly vertical circulation of air and means for relatively rotating each perforated plate with respect to the corresponding collection disk.
The operating mode of this known apparatus is based on the granulometric separation of aerosols, as a function of their kinetic energy, i.e. their inertia.
According to the prior art, the function of such an apparatus is the recovery of an adequate quantity of aerosols in order to have the necessary sensitivity for the exploitation of the results of the weighing operations, chemical analysis, counting the radioactivity or the X-fluorescene, etc. One known apparatus of this type is called the Andersen sampler, which comprises a stack of cylindrical hollow stages in which the aerosol circulates, each stack has a fixed plate perforated with calibrated holes and a collection disk, positioned downstream of the perforated plate, on which the particles are deposited by inertia.
Initially, an apparatus of the aforementioned type was used, whose collection disk was fixed. The deposit which collected on the collection disk was then localized on a very small surface area substantially corresponding to the projected surface of the holes formed in the perforated plate. This apparatus suffered from three main disadvantages which limited the quantity of aerosols collected, namely (a) the modification of the impaction characteristics of a given stage when the thickness of the deposit increased, (b) the reentrainment of aerosols from one stage towards the lower stages in the case of an excessive increase in the thickness of the deposit and (c) the auto-absorption caused by superimposing particles in the case of counting .alpha.-radioactivity or the deterioration of the emission spectrum of the radiation.
In order to obviate the aforementioned disadvantages, a collection method consisting of displacing the collection plates has already been envisaged, the collection disk rotating about its axis, which axis coincides with the axis of revolution symmetry of the cylindrical impacter. In this case, on the collection disks there are circular deposits of particles whose width is substantially equal to the projected surface of the holes formed in the perforated plate. Such a deposit reduces the disadvantages referred to hereinbefore, but fails to eliminate them.